1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Variable data printing for small lots and multiple products has undergone a shift from conventional offset printing to printing using electrophotographic image forming apparatuses. This shift requires the image forming apparatuses to provide the same level of image quality and registration accuracy as those of offset printing machines.
In image forming apparatuses including fixing units, sheets are sometimes expanded and shrunk due to fixing. Specifically, an image magnification varies entirely in a main-scanning direction and a sub-scanning direction due to fixing temperature, fixing pressure, and sheet moisture content, for example. Particularly, in the main-scanning direction, the width of a discharged sheet differs in the leading end and the trailing end thereof in some cases because the sheet is subjected to fixing and pressing while tensional force is applied in the width direction of the sheet to prevent the sheet from being wrinkled. As a result, deformation of an image occurs in a page of the sheet in some cases. In other words, a phenomenon of changing in image magnification occurs in some cases. In addition, the change differs in the front and the back sides of the sheet, thereby influencing the image quality of output images and the registration accuracy.
A technique has been known that detects a deformation of a sheet or an output image and corrects the image magnification on the basis of the detected deformation.
In a conventional manner, the image magnification is corrected by inserting pixels in image data or removing pixels from the image data on a pixel basis when an image is formed. In such a correction, a correction unit is 42 μm when the image resolution is 600 dpi while the correction unit is 21 μm when the image resolution is 1200 dpi, for example. It is apparent that the correction performed by the insertion or removal of pixels on a pixel basis under such image resolution causes visual noises such as moire and banding to be noticeable.
The correction performed on a pixel basis requires that the image resolution is equal to or larger than 2400 dpi in the main-scanning and the sub-scanning directions. In this case, image data amount in forming an image is 16 times larger than when the image resolution is 600 dpi in the main-scanning and the sub-scanning directions, and 4 times larger than when the image resolution is 1200 dpi in the main-scanning and the sub-scanning directions, thereby causing a problem in that the number of data buffers such as memories increases and the data needs to be processed at higher speed.
It is indeed necessary to correct the image magnification in the sub-scanning direction by the insertion or removal of pixels on a pixel basis under a high image resolution. In contrast, it is not necessary to correct the image magnification in the main-scanning direction under a high image resolution.
Japanese Patent No. 3918919 discloses image magnification correction with reference to a specific example in which the image resolution is 2400 dpi in the main-scanning and the sub-scanning directions. To cancel a deformation occurring in forming an image on a sheet, provided are an image storage unit that stores therein input image data, an image analysis unit that analyzes a deformation of an output image, a correction data generation unit that cancels the deformation occurring in the output image, and an image correction unit that performs correction processing on the input image data, and the image correction unit adds pixels to one side of the image data, inserts pixels in the image data with appropriate intervals, or adds the pixels to both sides of the image data. Japanese Patent Application Laid-open No. 2007-174060 discloses image magnification correction in which high resolution equal to or larger than 2400 dpi is required.
Those correction manners, however, do not solve the problem of the need to increase the number of data buffers such as memories and to process data at higher speed.
There is a need to provide an image forming apparatus that can adjust a change in the image magnification without using high-resolution data.